Pierce nuts generally include a projecting central pilot portion which pierces the metal panel or plate to which the pierce nut is to be attached and the pilot portion is then received through the pierced panel opening. The nut is then attached to the metal panel by a die member which forms a mechanical interlock between the nut and the panel. The panel may be deformed by the die member into grooves in the nut and/or the nut may be deformed to entrap the panel metal.
Many pierce nuts are used by the automotive industry to assemble cars in which many components of various kinds are attached to metal plates or panels. Pierce nuts are used to attach lamps and sheet metal parts, for example, to the vehicle. When such parts are attached, screws or bolts are threaded into the threaded hole in the pierce nut and the bolt or screw is tightened with rotating tools, such as a torque wrench, at prescribed torque values. The pierce nut must therefore have anti-torque or rotation resistance (the force that keeps the pierce nut from rotating within the metal plate when a bolt is threaded into the pierce nut and tightened) to bind it to the metal plate. After a component is attached to the pierce nut on a metal plate, external forces, such as vibration and tensile forces applied to an automobile, act upon the pierce nuts from the pull-through direction so as to pull them from the metal plate to which they are attached. Therefore, each pierce nut must have sufficient pull-through resistance (the force that keeps the pierce nut from coming out of the metal plate when the pierce nut and a bolt are engaged and the force is applied to the bolt perpendicular to the metal plate) that is stronger than these external forces.
As stated, the torque value of the rotating tool or torque wrench is predetermined, such that the rotation resistance of the pierce nut in the panel should be sufficient to resist this torque value, but the external forces applied to an automobile often cannot be forecast. Therefore, the aforementioned pull-through resistance must be relatively high.
When pierce nuts are being driven into metal plates, the pierce nuts are supplied to the installation tooling continuously through an outlet of a supply device, such as a hopper. Thus, it would be preferred if the shape of the pierce nuts permits free variance of the attachment direction on the surface of the metal plate. In other words, the shape of the pierce nut should permit free variation of the direction that each pierce nut emerges from the outlet of the hopper. In cases in which pierce nuts are to be driven into a metal panel in a number of locations, the pierce nut should be shaped so that the direction of the pierce nut outlet can be freely varied to suit the installation operation.
Pierce nuts have been used commercially in mass production applications for many years. More recently, U.S. Pat. No. 4,690,599 (Japanese patent opening 1983-109710) proposes a square pierce nut having a pilot (FIG. 1, Reference No. 3) used to shear and pierce metal plates. The outside surface of the pilot (Reference 4) is tapered inwardly to improve the pull-out resistance of the pierce nut because the panel metal is forced beneath the tapered wall as shown in FIG. 6. Also, the comers of the nut include a seat face 9 which supports the panel and an inclined side wall 10 which strengthens the interconnection between the panel and the opposed inclined walls.
Even more recently, a cylindrical pierce nut was proposed in Japanese Publication No. 1990-25049. The disclosed pierce nut includes a cylindrical central pilot (Reference 2), which during installation pierces a circular slug from the panel, and the nut includes a circular groove (Reference 6) adjacent the cylindrical pilot. An annular outside wall (4) defines the outside wall of the circular groove which is inclined inwardly to define a restricted opening which improves the pull-out resistance of the pierce nut in the metal plate material which is deformed into the groove. Radiating ribs or irregularities are formed on the panel supporting surface of the annular wall to improve the rotation resistance of the pierce nut in the panel.
The pierce nut proposed in Japanese patent opening 1983-109710 provides sufficient rotation resistance for most applications when sufficient panel metal is forced into the space between the pilot and the outside corners beneath the inclined surface 10. However, the pull-out resistance of this pierce nut depends entirely upon the amount of panel metal forced beneath the inclined side walls as shown in FIGS. 6 and 7. When the pierce nut is attached to a particularly thin metal plate or panel, very little material is forced into this tapered area, with the result that the contact surface between the outside surface area and the panel metal is very small, reducing the pull-out resistance value of the pierce nut. The published value for the pull-out resistance of the pierce nut shown in U.S. Pat. No. 4,690,599 is 83 kg for a 0.6 mm plate, 106 kg for a 0.8 mm plate and 199 kg for a 1.2 mm plate. Further, because the outside configuration of this pierce nut is almost square, it is not possible to freely alter the direction of the pierce nut from the outlet of the hopper when the direction of the pierce nut attachment to a panel is fixed.
The outside configuration of the pierce nut proposed in Japanese Publication No. 1990-25049 is cylindrical, such that it is possible to freely vary the direction of the pierce nut outlet of the transport device. Further, because panel metal is forced beneath the inclined surface of the external cylinder (FIG. 1, Reference No. 4), substantial panel metal is forced beneath this surface when the panel is thick, strengthening the pull-out resistance of the pierce nut. The published pull-out resistance for this nut is 250 kg with a copper plate having a thickness of 1.6 mm. With thin plates, however, very little panel metal is deformed beneath the inclined surface of the outer wall, substantially lowering the pull-out resistance for this nut, although it is not as low as the pierce nut described above. The advertised pull-out resistance for this nut is 168 kg with a 0.6 mm plate and 208 kg when a 0.8 mm plate is used. Because panel metal is deformed into the radial ribs or irregularities on the panel supporting surface of the outer cylinder, the rotation or torque resistance of the pierce nut is improved. However, this improvement in rotation resistance is substantially greater when a relatively thick plate is used, but the torque resistance falls significantly when the nut is attached to a thinner panel because the contact surface area is reduced. The advertised torque resistance is 255 kg-cm with a 1.6 mm plate, 145 kg-cm with a 0.8 mm plate and 137 kg-cm when the nut is attached to a 0.6 mm plate.
In the automotive industry, which utilizes many pierce nuts, there is a trend toward thinner metal panels and plates to reduce the weight of each car. Thus it is necessary to have pierce nuts shaped to provide the necessary rotation resistance and greater pull-out and pull-through resistance, even when used on thin metal plates. When, for example, it is necessary to achieve pull-out resistance in excess of 200 kg and sufficient rotation resistance to withstand the tightening torque applied by a torque wrench with a 0.6 mm plate and the bolt or screw meets resistance during engagement with the nut, existing pierce nuts of the types described above cannot consistently satisfy these requirements.
As described above, a pierce nut is typically attached to a metal panel or plate in conjunction with an installation die commonly referred to as a die button. The die button includes one or more projecting lips or protrusions configured to be received in the pierce nut groove or grooves. Where the pierce nut has an annular groove, the die button includes an annular lip or protrusion configured to be received in the annular groove of the nut. Where the self-attaching nut is a pierce nut, the die button typically includes a shearing edge or surface which cooperates with the outside surface of the pilot portion of the pierce nut to pierce an opening in the panel. The pierce nut pilot is then received through the pierced panel opening and the lip or protrusion then deforms the panel into interlocking relation with the nut groove or grooves. However, as described above, this mechanical interlock must be sufficient to withstand the torque which may be applied to the nut when a bolt is cross-threaded in the nut and tightened and the nut must have sufficient pull-out and pull-through resistance for commercial applications.
With existing die buttons, the material around the pierced panel opening is deformed by the two surfaces formed by the cylindrical outer surface of the circular lip or protrusion on the die button and the circular back face that is perpendicular to this outer surface and by the outside wall of the circular groove in the pierce nut when the panel metal is inserted into the annular groove. Thus, when insufficient panel metal is deformed by the annular lip of the die button, insufficient panel metal is inserted into the groove and it is not possible to increase the mechanical interlock between the panel metal and the groove to achieve the required pull-out strength. When the panel or plate is particularly thin, the volume of panel metal deformed into the groove is so low that the nut falls off the plate.
Thus, there remains a need for a self-attaching fastener which provides sufficient torque resistance and pull-out/through resistance for automotive applications, particularly for attachment to relatively thin metal panels. The most preferred embodiment is cylindrical, such that reorientation of the nut is not required when the nut is fed to alternative or multiple installation applications. Finally, there remains a need for an improved self-attaching nut installation die button which deforms sufficient panel metal into the nut groove to achieve the required pull-out resistance with a range of panel metal thicknesses, including thin panels.